This invention relates to controlling the relative humidity within a substantially sealed container, such as a package of food or a pack of cigarettes. More particularly, this invention relates to a device for inclusion in a substantially sealed container for maintaining a desired degree of relative humidity within the container.
Many products are packaged today within a transparent film overwrapping the entire package. This overwrap film has several purposes, but one of its most important functions is to act as a moisture barrier. Certain products--among them being foods and tobacco products--need to have a particular moisture content in order to be satisfactory to the consumer. If the product is too wet or too dry, it may convey a negative impression to consumers. The manufacturer can easily set the moisture level in the product at the factory, but must then depend on the overwrap film to keep moisture in or out of the package as needed until the product is consumed.
It is difficult to make a perfect moisture barrier with a typical film made from commercially available polymeric materials such as polyethylene, polypropylene, nylon-6, nylon-66, polyvinyl chloride, polyvinylidene chloride, or cellophane. There are two basic effects which prevent a perfect moisture barrier from being formed. First, the films covering packages may be imperfectly sealed in a manufacturing process. Secondly, polymeric films may not be totally impermeable to moisture vapor. That is, the moisture vapor may pass directly through the film as well as through poor seals.
It is desirable to have a practical means of maintaining a particular set relative humidity (RH)* or water activity (A.sub.w)** inside a package from the time it leaves the manufacturing plant until it is opened by the consumer. This way the product in the package will reach the consumer with the proper moisture content. * RH, or "relative humidity", is the amount of water vapor present in air at a particular temperature, expressed as a percentage of the total amount of water vapor which can be in the same quantity of air at that temperature. ** A.sub.w, or "water activity", is equal to RH/100--the decimal equivalent of RH.
Any system for controlling the relative humidity in a package must be able to cope with both moisture absorbed into the package from a very humid external environment, and with loss of moisture from the package into a very dry external environment.
Known methods for controlling the relative humidity in a package, or supplying moisture to the contents of a package, include putting into the package an absorbent material, such as blotter board, impregnated with water or other materials, so that the absorbent material will release its contents over time into the interior of the package. Another known method is to include in the package a pouch of cellophane or other porous or microporous cellulosic or polymeric membrane. The pouch encloses a hydrated salt which releases water vapor over time through the membrane into the pack.
These methods do not work well. The first method--putting wet blotter board, or some other water carrier into the package--simply puts excess water into the package or container. Initially the contents of the package will have too high a moisture level, and then, as the excess water is lost from the package, the contents will dry out. This method provides no means of stabilizing the relative humidity within the package at a desired level.
The second method--putting a hydrated salt inside a package--does give a buffering effect which helps stabilize the relative humidity in the package at a particular level. However, most hydrated salts establish an equilibrium relative humidity which is wrong--usually much too high for most packaging applications, and certainly for foods and for tobacco products. Furthermore, their RH buffering capacities per unit weight are low.
While these known practices are sufficient to prevent the contents of the cigarette pack from drying out for some period of time, until the water in the absorbent material or the hydrated salt is exhausted, there has not been any way to maintain the relative humidity within the pack at a specified desired level. Because cigarette packs are generally sealed in a polypropylene or other polymeric overwrap, when using the known practices the absorbent material or the hydrated salt will give up water to the interior of the pack until some equilibrium, dependent on ambient conditions, is reached. When water vapor leaks out through imperfections in the sealed wrap, additional water is given up by the absorbent or the hydrated salt, until all available water has been given up. Because the relative humidity set this way in the pack may be too high or too low, the cigarettes in the pack may consequently be soggy or dried out.
It has long been known that the equilibrium relative humidity over a hydrated salt or a saturated aqueous salt solution is a function of the temperature and of the hydrated salt or saturated salt solution. Each hydrated salt or saturated salt solution gives a discrete relative humidity at a given temperature. These have long been used as buffering devices to control the relative humidities of closed systems. However, there are some very important practical disadvantages to the use of hydrated salts and saturated salt solutions in the control of relative humidity.
Hydrated salts all have very low buffering capacities. In a truly closed system, this is not too important, but in a system which leaks, or which is enclosed partially or completely by a barrier which is somewhat permeable to water vapor, it is a very important practical issue. Large masses of a hydrated salt might be required to successfully buffer a package enclosed in a typical film such as polypropylene, polyethylene, nylon, cellulose, etc. Usually, the amount of hydrated salt required makes it an impractical medium for controlling relative humidity in a commercial package.
For use in consumable products, such as food or tobacco, many hydrated salts cannot be considered because of undesirable properties of the salt. For example, they may be toxic, or may create off-tastes in foods. Some may undergo chemical reactions with the other substances in the package. Consequently, the number of hydrated salts which can be practically considered is quite limited.
It is also quite difficult to find a hydrated salt which is acceptable for practical application, and which also produces the desired equilibrium relative humidity. And finally, the capacity of hydrated salts to control relative humidity is low. That is, a fairly large amount of hydrated salt is required to maintain control of the relative humidity for even a short period if the ambient conditions are very far from the target conditions.
Saturated salt solutions do not have the same capacity problem as do hydrated salts. There can be a great deal more water per unit volume or per unit weight in a saturated salt solution than in a hydrated salt. Furthermore, it is possible to adjust the initial ratio of excess salt to water, depending on whether the most probable problem expected is that the package will gain water or lose water.
There are also more salts which can be used to make saturated salt solutions, and their properties are well known. Even so, all ranges of relative humidities are not covered, and it is not always possible to find a saturated salt solution which will give exactly the equilibrium relative humidity needed.
Solutions which are saturated in two or more salts give equilibrium relative humidities which are different from those of saturated solutions of the original salts separately. Unfortunately, the equilibrium relative humidity over a solution saturated in two salts cannot be related in a linear manner to the equilibrium relative humidities of the saturated solutions of the individual salts. The interactions of the salts in solution are complex, and the equilibrium relative humidity over a solution saturated in two salts is not readily predicted.
In addition, like hydrated salts, many salts whose saturated solutions give desirable equilibrium relative humidities cannot be used because of other properties of the salts such as toxicity, off-taste problems, induced corrosion, chemical reactions, etc., as discussed above.
A further consideration is the need to contain a solution inside the package in such a way that it can equilibrate with the atmosphere inside the package, and at the same time not spill into the rest of the package, nor wick into the package or its contents. Obviously, an open container cannot be used, and a closed container would not allow equilibration with the atmosphere inside the package.
It would be desirable to be able to provide a device which will buffer the relative humidity in a closed container such as a sealed package of food or a sealed cigarette pack.